Variable frequency high sensitivity microwave spectroscopy



May 6, 1969 M. BRINKERHOFF 3,443,217 VARIABLE FREQUENCY HIGH SENSITIVITYMICROWAVE SPECTROSCOPY Filed Aug. 2, 1966 r l2 l3 MICROWAVE CRYSTALGENERATOR WAVE GU'DE DETECTOR 2i f HIGH FREQUENCY A C SOURCE OUT PUT ICIRCUIT OUT PUT #24 .I? 3 l4 2 I2 FIG. 2

3| 32' as MICROWAVE CRYSTAL GENERATOR WAVEGU'DE DETECTOR FIELD CIRCUITSTARK +v (O) SEPTUM VOLTAGE-V TIME F 9 INVENTOR JORIS M. BRINKERHOFF BYAW ATTORNEY Sheet 0192 y 1969. J. M. BRINKERHO'FF 3,443,217

VARIABLE FREQUENCY HIGH SENSITIVITY I MICROWAVE SPECTROSCOPY Filed Aug.2. 1966 Sheet 3 of 2 c 35? 25 GENERATOR 1; :1

FIG. 6 5o FROM PHASE I COUNTER TUNED o E I\ I s I I E vco I AMPLIFIERCOUNTER FROM 52 v HIGH FREQUENCY 53 L49 STARK 55 FIG. 7 FIELD SWITCHFRoM TUNED PHASE PHASE AMPLIFIER sENsITIvE A SENSITIVE ouTPuT DETECTORDETECTOR FROM 1 A HIGH FREQUENCY L L Ac VOLTAGE SOURCE M FRoM STARKFIELD 0, I5 CIRCUIT FIG. 8 INVENTOR.

JORIS M. BRINKERHOFF BY ATTORNEY United States Patent U.S. Cl. 32458.5 7Claims ABSTRACT OF THE DISCLOSURE A variable frequency microwavegenerator coupled to a Waveguide containing a Stark septum andterminating in a crystal detector for obtaining a spectroscopic analysisof gas contained within the waveguide. High and low frequency modulationof the signal passing through the waveguide identifies with improvedaccuracy the resonant absorption peaks of the gas.

This invention relates in general to microwave spectroscopy and moreparticularly to apparatus for high sensitivity microwave spectroscopy.

In microwave spectroscopy the molecular composition of a gas is studiedby determining the microwave frequencies at which absorption peaksoccur. The basic apparatus which has been used for such study consistsof a variable frequency microwave generator, a microwave crystaldetector and a waveguide coupling the two together with the waveguidecontaining the gas to be analyzed. The absorption peak is thendetermined by scanning the frequency of the microwave generator acrossthe range of interest and measuring the variations in microwave power atthe detector. Inasmuch as a typical value for the reduction of theabsorption peak is around 10 for each centimeter of waveguide length,extreme sensi tivity of measurement is required. Thus, in a waveguide of100 centimeters length the reduction would amount to only around or0.1%. The problem is compounded when the purpose of the study is todetermine parts per million of one gas within another gas, since a 10part per million concentration, for example, would typically result inan absorption of only around 10 or one millionth of 1%, in a one-meterwaveguide.

Since the measurement under discussion is that of the comparison ofmicrowave power received when an absorption peak is present to that ofthe power received without any absorption peak, then any variation inthis ambient power level constitutes a noise background for themeasurement. There are several sources of such noise. One source is thenoise from the crystal detector itself. This noise generally decreaseswith increasing frequency and is generally referred to as l/ noise.Another source of noise is variation in power level at the microwavegenerator (oscillator noise), while still a third is the variation intransmission efficiency of the microwave guide as a function offrequency.

In the past there have been a number of approaches to the problem ofreducing the noise for microwave spectroscopy measurements. One suchapproach consisted of frequency modulating the microwave source andemploying a tuned circuit such as a tuned amplifier on the output of thedetector so that the system responded only to frequencies at and aboutthe modulating frequency. Under these circumstances, the reduction ofl/f noise and noise due to power fluctuations of the generator led to anincrease in signal to noise ratio. In a more refined system, the outputsignal from the amplifier was rectified (in some cases, in synchronismwith the modulation frequency),

and the resulting direct current voltage level, which then constitutedthe output signal from the device, was averaged over a relatively longtime period (e.g., 1 second). In order to determine the location ofabsorption peaks, the frequency of the microwave generator was scannedslowly across the range of interest and the output from the rectifierwas displayed on a chart recorder to provide a visual waveform. Theprincipal difiiculty with this system lay in the fact that the frequencymodulation employed resulted in the generation of numerous spurioussignals due to the variation of transmission efiiciency of the systemwith frequency.

In another approach, a thin electrically conductive plate was mountedcentrally within the waveguide and extended longitudinally along thewaveguide. This plate was insulated from the waveguide and sufiicientvoltage (typically, around 1000 volts) was applied between thiselectrode and the waveguide to create an electrostatic field ofsufficient strength to produce the Stark effect. In the Stark effect,the presence of a high electrostatic field causes a splitting of theabsorption peak. For example, if

the absorption peak were at a frequency f then the presf ence of asufficient electrostatic field would create a pair of absorption peaksof reduced amplitude, one at a lower frequency than f and one at ahigher frequency, and the original peak at f would have disappearedalmost completely. (Because of this Stark effect, the central electrodeused in this configuration of waveguide has become known as a Starkseptum.) In this mode of operation the voltage creating an electrostaticfield was applied at some rela tively high frequency between the Starkseptum and the Wall of the waveguide, and the output circuit from thecrystal detector was tuned to this frequency. This output was thenrectified as described above. The direct current amplitude at the outputthus represented the difference between microwave absorption with andwithout the electrostatic field. Accordingly, the signal amplitude didnot include, at least primarily, variations due to variations intransmission efficiency of the waveguide. A difiiculty that stillremained with this system, however, although of second order, arose fromthe fact that the Stark modulation was likely in many cases to produce asmall modulation of the transmission characteristics of the waveguidesystem. Additionally, in certain cases where broadband electrical tuningwas required, the microwave generator usually employed was a backwardwave oscillator. When this generator is used in conjunction with a highfrequency Stark field, radiation from the Stark field is likely tofrequency modulate its output. This modulation introduces variations inthe microwave signal received at the detector due to variations oftransmission characteristics with frequency. To obviate suchdifiiculties, which were particularly pronounced at low signal levels,the frequency of the microwave generator was slowly varied over anappropriate range so as to permit visual recognition of the waveform.This range normally included the frequencies at which the displacedpeaks would occur as well as the original frequency of the absorptionpeak so as to provide for a more unique criterion of signal recognition.

Another problem which was common to both the frequency modulation andStark modulation systems arose when it was attempted to use exceedinglylong time constants on the output of the rectifier in an attempt to in-1 crease the signal-to-noise ratio. This was impractical bebecause ofthe fact that the rectified output would be likely to drift slowly withtime so that in many cases this slow drift would becomeindistinguishable from the waveform which was to be generated by a slowfrequency scan over the absorption peak. In a more refined system thisdifficulty was obviated to some degree by conversion of the DC level atthe output of the rectifier to a pulse repetition rate. As the DC levelwould vary the pulse repetition rate would vary. The pulse output wasthen applied to a channel of a multichannel digital storage system. Themicrowave frequency was then varied relatively rapidily, i.e., beforethe ambient level at the output of the rectifier would have a chance tochange significantly. In synchronism with the variation in microwavefrequency, subsequent channels of the multichannel storage system wouldbe addressed. This procedure was then repeated many times over so as toprovide a long time constant averaging of the waveform. The difiicultyassociated with this system was that a considerable portion of themeasurement time was spent in making measurements at frequencies wherethe signal-to-noise ratio was very low.

It is therefore a primary object of the present invention to provideapparatus for microwave spectroscopy for accurate and efficientdetermination of absorption peaks.

It is another object of this invention to provide apparatus for micowavespectroscopy in which noise in the output signal is significantlysuppressed.

It is still another object of this invention to provide apparatus formicrowave spectroscopy in which the generation of spurious signals byStark modulation are suppressed.

It is yet another object of this invention to provide apparatus formicrowave spectroscopy wherein provision is made for the use of longtime constants without the necessity of making measurements at pointsother than at the absorption peak.

Broadly speaking, in the spectroscopy system described herein, thespectrometer is formed with a variable frequency microwave generatorcoupled to a waveguide containing a Stark septum and terminating in acrystal detector. An alternating current signal at a fixed, relativelyhigh, frequency is coupled to the spectrometer to vary the microwavepower received by the crystal detector in accordance with the microwaveabsorption characteristic of the particular gas within the waveguide atthe frequencies of operation of the microwave generator. As will appearin more detail below, this variation may he accomplished in either oftwo ways, one, by using the AC signal to frequency modulate themicrowave generator or, alternatively, by using the AC signal to varythe voltage applied between the Stark septum and the wall of thewaveguide. A tuned amplifier at the output of the crystal detector istuned to this relatively high frequency and the phase-sensitive detectorwhich is used to rectify the output from the amplifier is synchronizedwith this AC signal. Since the amplifier passes only frequencies in arelatively narrow bandwidth around the high frequency, there isconsiderable reduction of the 1/) noise and oscillator noise.

In addition to this high frequency variation, a second much lowerfrequency component is provided in the output signal. This component isgenerated either by a switch operated at predetermined intervals, suchas one second intervals, or by a relatively slow AC source. In the casewhere the high frequency variation is introduced by frequency modulationof the microwave generator, the low frequency variation is introduced byvarying the voltage applied to the Stark septum. In this instance, theoutput from the high frequency phase detector is a voltage level varyingat the low frequency. Alternate halves of this resulting voltagewaveform are indicative of signal plus noise, while the other halvesrepresent noise lone. However, both halves include the ambient D.C.level of the rectifying circuit. The alternate outputs may bedigitalized and accumulated separately in a pair of scalers to determinethe ratio of the signal to noise, or alternatively, a second phasedetector synchronized with the low frequency signal source may becoupled to the output of the first phase detector. The DC output fromthis second phase detector is then directly proportional to thedifference between the modulated signal with and without the Starkeffect, Since this arrangement permits a direct de- 4 termination of themicrowave power level with and without a Stark field, the microwavegenerator may be maintained fixed at the frequency of the absorptionpeak for a gas of interest, without the necessity of sweeping thefrequency to determine the waveform.

In the second case, when the high frequency variation is introduced byapplication of an alternating Stark field, the low frequency componentis provided by low frequency reversal of the phase of the Stark fieldwith respect to the source of the high frequency modulation signal. Thischange of phase results in a change of polarity of the D.C. outputsignal. Since only the Stark field has changed phase, then, in theresultant low frequency waveform, the ambient level at the output of thedetector first adds, then substracts from the signals and is thereforecancelled out. This output may be treated in the same fashion as thatfor the first case.

Other objects and advantages will become apparent from the followingdetailed description when taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is an illustration in block diagram form of a spectroscopyapparatus constructed in accordance with the principals of thisinvention;

FIG. 2 is a plan view of a waveguide element suitable for use in thepractice of this invention;

FIG. 3 is a cross-sectional view taken along the line 33 of FIG. 2;

FIG. 4 is a cross-sectional view taken along the line 44 of FIG. 3;

FIG. 5 is an illustration in block diagram form of a second embodimentof a spectroscopy apparatus constructed in accordance with theprincipals of this invention;

FIG. 6 is an illustration partially in block diagram and partially inschematic form of a circuit for producing an electrostatic field withina waveguide, which circuit is suitable for use in the practice of thisinvention;

FIG. 6A is an illustration in schematic form of a switching arrangementwhich may be substituted for the low frequency AC generator shown inFIG. 6;

FIG. 7 is an illustration in block diagram form of output circuitrysuitable for use in a spectroscopy apparatus as illustrated in eitherFIG. 1 or FIG. 5;

FIG. 8 is an illustration in block diagram form of another embodiment ofan output circuit suitable for use in the spectroscopy apparatus asillustrated in either FIG. 1 or FIG. 5; and

FIG. 9 is a graphical illustration of waveforms helpful to anunderstanding of this invention.

With reference now to FIG. 1, a microwave generator 11 is coupled to oneend of a waveguide 12, the other end of which is coupled to a crystaldetector 13. The microwave generator may be a typical commerciallyavailable generator operating in the K band or K band and would mostusually be a backward wave oscillator such as a Varian Associates ModelVA-163. The waveguide 12 is constructed generally as an elongatedcylinder of perhaps two meters length having a rectangular cross sectionwith the cross section dimensions typically being 0.4 inch by 0.9 inch.Further details of the waveguide construction are illustrated in FIGS.2, 3 and 4. The waveguide includes at either end a flange portion 17 andthe openings at each end are sealed off by mica windows 18 so that theoverall waveguide may be considered as gas tight. As above mentioned,the purpose of this spectroscopy apparatus is the analysis of themicrowave absorption characteristic of a gas. The waveguide 12 thenserves as the test chamber in which microwaves are transmitted throughthe gas to be analyzed. This waveguide is evacuated through port 27, andafter evacuation the test gas is supplied to the waveguide 12 throughthe same port 27. The waveguide is formed, as illustrated in FIGS. 3 and4, wlth a Stark septum 25 bisecting it longitudinally. The Stark septumis formed as a thin conducting plate supported within the waveguide oninsulators 26, which typically would be formed of Teflon rails. TheStark septum at either end is narrowed down to a point in order tominimize end effects. An electrical connector 14 provides for electricalcontact to the Stark septum insulated from the waveguide 12. Thedetector 13 coupled to the output side of the waveguide is aconventional microwave is a conventional microwave crystal detector suchas a IN26C.

Referring again to FIG. 1, the output from the detector 13 is shownconnected to a tuned amplifier 14, the output of which is in turnconnected to an output circuit 15. The microwave generator 11 hasapplied to it a high frequency AC modulating signal from AC source 21and this same signal is provided to the output circuit 15. The tunedamplifier 14 is tuned to this same frequency; the value of the frequencyis selected to be sufficiently high so that the 1/ f noise is very low.A typical value is 100 kc./sec. An amplifier bandwidth of about kc./sec.for this frequency is satisfactory. A field circuit 22 for providing aStark field is coupled through electrical connector 14a to the Starkseptum 25 and this field circuit 22 also provides a signal to outputcircuit 15. The field circuit 22 provides for the application of anelectrostatic field between the Stark septum 25 and the wall of thewaveguide 12. This field is arranged to be of sufficient magnitude,typically 1000 volts, to produce the Stark effect on the absorption peakof a gas which is to be analyzed. In its simplest form, the fieldcircuit 22 for this apparatus would consist of a square wave generatorcapacitatively coupled to the Stark septum and operated at a suitablefrequency, that is, one which is relatively low compared to that of thehigh frequency AC source 21, and yet one in which the time period isshort compared to the traversal time of the generator 11 over afrequency absorption line. In one mode of operation this field circuit22 might operate at a frequency of 100 cycles per second. As aboveindicated, the amplitude of the waveform applied to the Stark septum 25would be in the order of volts and a signal at this same frequency, butof lesser amplitude, is supplied to the output circuit 15.

The exact configuration of the output circuit will depend upon the modeof operation of the overall system. Thus, the system may be operatedeither by maintaining the microwave generator 11 at a frequencycorresponding to an absorption peak frequency for the gas to be analyzedor alternatively this frequency may be swept slowly over a range ofinterest in order to provide an actual measurement of the waveform ofthe absorption peak. In the latter case, that is, where the microwavegenerator 11 is to be swept slowly over a range of frequency, outputcircuit 15 is generally of the form illustrated in FIG. 8. As shown inFIG. 8, the output from the tuned amplifier 14 is rectified by beingapplied to a phase sensitive detector 61 which is synchronized by thesignal from the high frequency AC source 21. The output then from thisphase sensitive detector 61 is in essence a rectified 100 kc./sec.signal. This output from the phase sensitive detector 61 is now appliedto a second phase sensitive detector 62, which is synchronized by the100 cycle per second signal from the field circuit 22. Hence the outputfrom this second detector is the rectified 100 cycle per secondcomponent of the output signal from the phase sensitive detector 61.

The operation of the microwave spectrometer illustrated in FIG. 1 is asfollows. The microwave generator 11 provides microwaves to the waveguide12, which microwaves are modulated at the high frequency of the ACsource 21, with the amplitude of this modulation being madeapproximately equal to the line width of the absorption peak. When thefrequency at which the microwave generator 11 is operated is equal tothe absorption frequency of the gas to be tested, this modulation thencauses the microwave power transmitted to vary periodically from a valueat one peak equal to the ambient microwave level to a value equal to themaximum absorption level at the other peak, with this variationoccurring at twice the modulation frequency. It should be noted thatwhen the frequency of operation of the microwave generator 11 is, forexample, A of a line width removed from the peak absorption frequency,this same variation in general occurs. However, in this instance afrequency equal to the modulation frequency itself becomes predominant.

In the specific descriptions of the apparatus given above, as well as inthose which will appear below, the amplifier is described as tuned tothe modulation frequency and the phase sensitive detector issynchronized by this same frequency. It is evident that those elementsmay instead be tuned to the second or higher harmonics of thefundamental frequency.

In addition to this high frequency variation there is a low frequencyvariation in transmitted microwave power. This low frequency variationresults from varying the Stark field which produces a variation from thepeak absorption level to the ambient level, with this second variationoccurring at a lower frequency, in this instance cycles per second. Theoutput signal from the tuned amplifier 14 then contains a 100 kc./sec.component due to the frequency modulation of the microwave generator 11and a low frequency modulation component due to the variation of theStark field. The output from the phase sensitive detector 61 (FIG. 8)is, in essence a 100 cycle per second wave of amplitude equal to thedifference between the microwave transmission with and without the Starkeffect. By applying this signal to the second phase sensitive detector62, which is synchronized by a signal from the field circuit 22, arectified output signal is produced in which the direct current level isnow proportional to the difference between the microwave powertransmitted with and without the Stark effect. By scanning the frequencyof the microwave generator 11 over the frequency range of interest, theoutput signal appearing on output 24 as a function of time will have thewaveform of the absorption peaks of the gases contained within thewaveguide 12.

The microwave spectrometer illustrated in FIG. 1 can also be operated inthe mode where the frequency of the microwave generator 11 remains fixedat a frequency corresponding to the absorption frequency of a gas whichis to be quantitatively determined. In this instance the field circuit22 wouldgenerally not be a conventional square wave generator but wouldrather be a 1000 volt supply which is alternately applied and removedfrom the Stark septum 25 by means of a motor controlled switch. In thisinstance the switch would operate at a value of, typically, oneswitching action per second, and the Stark field would therefore beapplied for one second, removed for one second, and so forth. In thisinstance the output circuit 15 would not correspond to the configurationillustrated in FIG. 8. A suitable form of output circuit for this modeis shown in FIG. 7. In this circuit the signal from the tuned amplifier14 is applied to a phase-sensitive detector 45 which is synchronized bythe signal from the high frequency AC source 21. The output from phasesensitive detector 45 is then applied to a voltage controlled oscillator51. The voltage controlled oscillator 51 generates output pulses at arepetition rate proportional to the DC voltage level at its input. Theoutput from the voltage controlled oscillator 51 is coupled through aswitch 54 to either one of these two sealers, and is actuated back andforth by means of a cam 53 operated by a motor 52.

This actuating cam 53 also controls the switching of the Stark fieldswitch 55. In this mode of operation the high frequency source againmodulates the frequency of the microwave power applied to the waveguide12; however, the variation in the Stark field occurs much more slowlythan the previous typical value of 100 c./s. Once again, the output fromphase-sensitive detector 45 is a signal which varies at the rate ofalternation of the Stark field and the amplitude of this slowly varyingcomponent is equal to the difference in microwave absorption with andwithout the Stark field. The output from the voltage controlledoscillator 51 is then a signal which alternates between two repetitionrates, one representing the microwave power transmitted with a Starkfield and the other representing the microwave power transmitted withoutthe Stark field. By synchronizing the action of the switch 54 with theaction of the Stark field switch 55, one scaler, e.g., 48, accumulatespulses representing the microwave transmission with the Stark fieldwhile the other scaler accumulates pulses representing the microwavetransmission without the Stark field. The apparatus may be run for aconsiderable period of time and the accumulated difference between thecounts on scaler 48 and scaler 49 represents this difference betweensignal and noise over a long integration period.

If the voltage controlled oscillator 51 is made to have a logarithmicresponse, i.e., wherein the logarithm of the output frequency varies asthe input voltage level, then it should be noted that the logarithm ofthe ratio of the counts accumulated on scaler 48 to the countsaccumulated on scaler 49 is now proportional to the difference in levelsand hence proportional to the magnitude of the microwave absorptionpeak. One advantage of such an arrangement is that timing errors may becorrected by reversing the phase of the slow Stark variation withrespect to the cam operated counter switch 54. By then taking thegeometric mean of the two ratios which are obtained, any systematicerror in the timing device is cancelled.

Referring now to FIG. 5, a second and preferred embodiment of themicrowave spectroscopy system is shown. In this system both the highfrequency and the low frequency variation in the microwave transmissionare accomplished within the Stark field circuit and there is nofrequency modulation of the microwave generator itself. A microwavegenerator 31 is coupled to a waveguide 32 which in turn is coupled to acrystal detector 33. The output of the crystal detector 33 is connectedthrough a tuned amplifier 34 to an output circuit 35. A field circuit 43is coupled to waveguide 32 and provides also two independent signals tooutput circuit 35. The microwave generator 31, waveguide 32, crystaldetector 33, and tuned amplifier 34 are all identical to the componentsused in the system illustrated in FIG. 1. The output 35 will againdepend upon the exact mode of operation of the system but will have thegeneral form illustrated in either FIG. 7 of FIG. 8. The field circuit43 in this instance differs significantly from the Stark field circuitused in the system of FIG. 1.

In FIG. 6 there is shown a configuration of the field circuit 43 whichproduces a Stark field and is suitable for operating the microwavespectroscopy system of FIG. in the sweeping mode, that is, where thefrequency of microwave generator 31 is swept over the range of interest.In the field circuit illustrated in FIG. 6 a square wave generator 41has one terminal capacitatively coupled through capacitor C to the Starkseptum 25 while the other terminal is connected to the wall of waveguide12. Square wave generator 41 would be a generator operating typically ata frequency of approximately 100 kilocycles/ sec. and providingsufficient output voltage to create the Stark effect within thewaveguide. This voltage would have a nominal value of 1000 volts peak topeak. Also included in this field circuit is a low frequency ACgenerator 47 which provides a voltage output of approximately 1000 voltspeak to peak but at a much lower frequency, for example, 100 cycles persecond. One terminal 49a of this low frequency generator is connectedthrough resistor R to the Stark septum while the other terminal 50 isconnected directly to the wall of the waveguide 12. The time constantformed by the combination of the resistor -R and the capacitor C may beany convenient value provided that it is somewhat less than thereciprocal of the frequency of the low frequency AC generator 47, whileat the same time the resistance R is generally chosen to be large incomparison to the driving impedance of square wave generator 41.

In operation, a 1000 volt signal is being applied between the Starkseptum and the wall of the waveguide 12 at a repetition rate ofapproximately 'kilocycles/ sec. At the same time the 1000 volt signal isbeing applied from the low frequency AC generator 47 at a 100 cycle persecond rate across the same two points. The effect of the low frequencysignal is to cause a phase shift in the Stark effect, that is to changethe phase at which the absorption peak is split.

Referring to FIG. 9, curve a represents the 100 kc./sec. waveform whenthe polarity of the 100 c./ s. waveform is positive. Under theseconditions the voltage on the Stark septum varies from zero to apositive value with respect to the waveguide wall. The Stark effectoccurs when this voltage is at +V. Curve b of FIG. 9 shows the 100 kc./sec. waveform when the polarity of the 100 c./ s. waveform is negative.The voltage on the Stark septum now varies from zero to a negativevalue. Since the Stark effect depends only on the presence of theelectrostatic field and is essentially independent of polarity, it nowoccurs when the septum voltage is at V. As indicated by the diagonaldotted line, the effect of the 100 cycle waveform is then to shift thephase of the 100 'kc. waveform at which the Stark effect occurs. If theoutput circuit is as shown in FIG. -8, and the first phase sensitivedetector 61 is synchronized with a signal from the square wave generator41, then its output will be a rectified 100 kc./sec. wave. The change inphase which occurs at the low frequency rate of 100 c.p.s. will causethe polarity of this rectified signal to vary at this low frequencyrate. Since the phase difference occurs only with respect to the Starkfield, then any ambient levels produced by 100 kc./sec. interference, oras for example, by slight frequency modulation of the microwaveoscillator, fail to produce variation on the rectified signal at thegiven low frequency rate. Ordinarily, any ambient level at the output ofthe phase-sensitive detector fails to produce such a low frequencyvariation except that which is due to the Stark effect itself.

The output of the first phase-sensitive detector is coupled to thesecond phase sensitive detector 62, which is synchronized with thesignal from the low frequency AC generator 47. The output from thissecond phase detector is then the rectified 100 c.p.s. signal, and theDC value of this signal is thus proportional to the signal amplitude ofthe difference between transmission with and without the Stark field.

When the spectroscopy system illustrated in FIG. 5 is operated in thesecond mode, that is with the frequency of the microwave generator 31maintained at a selected value corresponding circuit 35 generally takesthe form illustrated in FIG. 7. In addition, the field circuit 43 issomewhat modified from the configuration shown in FIG. 6. In this modethe field circuit 43 does not include a low frequency AC generator 47but rather a switching arrangement is connected between terminals 49 and50. This switching arrangement may conveniently take the form shown inFIG. 6A in which a double pole, double throw switch 55 and a voltagesource 57 are interconnected to form a circuit which provides a voltagebetween terminals 49a and 50 with the polarity of this voltage beingreversed by changing the position of the arm of the switch 55. In thismode, the operation is substantially the same as that for the sweepingmode in that the reversal of polarity of the voltage applied betweenterminals 49a and 50 changes the phase of the variation of the Starkeffect with respect to the high frequency produced by square wavegenerator 41. The output circuit illustrated in FIG. 7 again provides asuitable form of output circuit for this second mode of operation. Inthis instance the cam actuator 53 would be used to effect the switchingchange in the double pole, double throw switch 55. The synchronizingsignal for the phase detector 45 is derived, as before, from the squarewave generator 41. A typical value 9 for the frequency with which theswitch is proximately one change per second.

The invention has been described in terms of two specific embodiments ofspectrometer systems employing the Stark effect. It is apparent that,while specific components have been shown for these illustrations, anumber of equivalent components may also be employed. Thus, in thenon-sweeping mode counters have been used as the integrating circuits atthe output. Many otherforms of integrating circui-ts are suitable suchas capacitator tanks and other memory devices which are suitable forrelatively long time integration. Similarly, a variety of components andarrangements may be utilized to perform the switching functions and togenerate the voltage and perform the readout of the DC levels. Theinvention has also been described in terms of systems which employ theStark effect. A spectrometer system incorporating these same principlesmay also be constructed using the Zeeman effect rather than the Starkeffect. In the Zeeman effect the splitting of the absorption peak isproduced by generating a suitable magnetic, rather than electrostatic,field. -In the Zeeman effect spectrometer the systems illustrated inFIG. 1 or in FIG. 5 may be used. In this case however the waveguidewould be formed of aluminum, or a glass tube with focussing electrodesand the magnetic field would be created with conventional means such asinduc- .tive loops. The field circuit 22, in the system illustrated inFIG. 1 would now be a low frequency current source supplying current toproduce the magnetic field. Similarly, in the system of FIG. 5, thefiield circuit 43 would be formed of high and low frequency currentsource coupled to an inductive coli. It may be noted that in all casesthe frequency absorption characteristic of the gas is varied at the lowfrequency to produce the low frequency change in microwave powerreceived at the detector.

Having described the invention, various modifications and improvementswill now occur to those skilled in the art and the invention disclosedherein should be considered as limited only by the spirit and scope ofthe appended claims.

What is claimed is:

1. Apparatus for measuring the microwave absorption characteristics of agas comprising,

a microwave generator for generating microwaves at a selected frequency;

a waveguide coupled to said microwave generator, said waveguide beingformed to constitute a chamber for containing gas to be measured;

a microwave detector coupled to said waveguide for measuring themicrowave power transmitted along said waveguide and providing anelectrical output indicative thereof;

a first means for producing a first variation with time in the microwavepower received by said detector, the magnitude of said first variationbeing controlled by the microwave absorption characteristic of said gasat said selected frequency;

second means for producing a second variation with time in the microwavepower received by said detector by changing the microwave absorptioncharacteristic of said gas, said first means providing a time variationat a first frequency and said second means providing a time variation ata second frequency substantially lower than said first frequency;

an amplifier coupled to said microwave detector, said amplifier beingtuned in a relatively narrow bandwidth around said first frequency; and

output means coupled to said amplifier for providing output signalsindicative of the microwave absorption characteristics of said gas;

said first means comprising a source of alternating current voltagecoupled to said microwave generator for frequency modulating saidmicrowave generator at said first frequency; and

said second means comprising means for generating within said waveguidean electrostatic field which actuated is apvaries at said secondfrequency from a minimum value to a maximum value where said maximumvalue is sufficient to significantly affect said absorptioncharacteristics of said gas within said waveguide at said selectedfrequency.

2. Apparatus in accordance with claim 1 wherein said waveguide is formedas an elongated chamber for transmission of microwaves along thelongitudinal axis thereof, and further including an electrodelongitudinally bisecting said waveguide, said second means being coupledbetween said waveguide and said bisecting electrode for producing anelectrostatic field between said bisecting electrode and said waveguide.

3. Apparatus in accordance with claim 1 wherein said second meansincludes a second source of alternating current voltage operating atsaid second frequency and wherein said output means comprises a firstphase sensitive detector coupled to said amplifier output, said firstphase sensitive detector being operated synchronously with said sourceof AC voltage within said first means, and a second phase sensitivedetector coupled to the output of said first phase sensitive detector,said second phase sensitive detector being operated synchronously withsaid source of AC voltage within said second means.

3. Apparatus in accordance with claim 1 wherein said second meansincludes a second source of alternating current voltage operating atsaid sceond frequency and wherein said output means comprises a firstphase sensitive detector coupled to said amplifier output, said firstphase sensitive detector being operated synchronously with said sourceof AC voltage within said first means, and a second phase sensitivedetector coupled to the output of said first phase sensitive detector,said second phase sensitive detector being operated synchronously withsaid second source of AC voltage within said second means.

4. Apparatus in accordance with claim 1 wherein said second meansincludes a switch, said switch being repetitively operated atpredetermined intervals for applying and removing said electrostaticfield within said waveguide, said predetermined intervals being suchthat said field is applied and removed at said second frequency, andwherein said output means includes a rectifying means coupled to theoutput of said amplifier, said rectifying means being synchronouslyoperated with said AC voltage source in said first means and wherein theoutput from said rectifying means is coupled to a first terminal whensaid switch is in one position and to a second terminal when said switchis in a second position.

5. Apparatus in accordance with claim 3 but including a voltagecontrolled oscillator of logarithmic characteristic coupled between saidswitch and said output from said phase sensitive detector whereby theoutput from said voltage controlled oscillator is coupled to a firstterminal when said switch is in one position and to a seccond terminalwhen said switch is in a second position.

6. Apparatus for measuring the microwave absorption characteristics of agas comprising:

a microwave generator for generating microwaves at a selected frequency;

a waveguide coupled to said microwave generator, said waveguide beingformed to constitute a chamber for containing gas to be measured;

a microwave detector coupled to said waveguide for measuring themicrowave power transmitted along said waveguide and providing anelectrical output indicative thereof;

a first means for producing a first variation with time in the microwavepower received by said detector, the magnitude of said first variationbeing controlled by the microwave absorption characteristic of said gasat said selected frequency;

second means for producing a second variation with time in the microwavepower received by said detector by changing the microwave absorptionchar acteristic of said gas, said first means providing a time variationat a first frequency and said second means providing a time variation ata second frequency substantially lower than said first frequency;

an amplifier coupled to said microwave detector, said amplifier beingtuned in a relatively narrow bandwidth around said first frequency; and

output means coupled to said amplifier for providing output signalsindicative of the microwave absorption characteristics of said gas;

said output means comprising a first detector means for rectifying thatfrequency component of said microwave detector electrical outputcorresponding to said first frequency and second detector means coupledto the output of said first detector means for rectifying that frequencycomponent of the output from said first detector means corresponding tosaid second frequency.

7. Apparatus for measuring the microwave characteristics of a gascomprising,

a microwave generator for generating microwaves at a selected frequencyformed as an elongated chamber for transmission of microwaves along thelongitudinal axis thereof and further including an electrodelongitudinally bisecting said waveguide;

a waveguide coupled to said microwave generator, said waveguide beingformed to constitute a chamber for containing gas to be measured;

a microwave detector coupled to said waveguide for measuring themicrowave power transmitted along said waveguide and providing anelectrical output indicative thereof;

a first means for producing a first variation with time in the microwavepower received by said detector, the

absorption magnitude of said first variation being controlled by themicrowave absorption characteristic of said gas at said selectedfrequency;

second means for producing a second variation with time in the microwavepower received by said detector by changing the microwave absorptioncharacteristic of said gas, said first means providing a time variationat a first frequency and said second means providing a time variation ata second frequency substantially lower than said first frequency;

said first means comprising a source of alternating current voltageoperating at said first frequency and means coupling said source ofalternating current voltage between said waveguide and said bisectingelectrode for varying the electrostatic field within said waveguide;

said second means comprising means for applying an alternating currentvoltage at said second frequency between said waveguide and saidbisecting electrode;

an amplifier coupled to said microwave detector, said amplifier beingtuned in a relatively narrow bandwidth around said first frequency; and

output means coupled to said amplifier for providing output signalsindicative of the microwave absorption characteristics of said gas.

RUDOLPH V. ROLINEC, Primray Examiner. P. F. WILLE, Assistant Examiner.

